invention relates to a vane assembly for a gas turbine engine of the type having concentrically arranged inner and outer shrouds with circumferentially spaced vanes therebetween.
Vane assemblies are needed in gas turbine engine compressors and turbines to ensure optimum inlet flow of the working fluid to the rotor blades. It is especially the vane assembly of the first uurbine stage directly downstream of the combustion chamber which comes under high thermal load on account of the elevated temperature and pressure. This is why the vane assembly must either come in a correspondingly heat-resistant material (ceramic) or be kept at low temperature by means of a cooling medium (air).
Vane assemblies of this description are either fully integral (single-piece) constructions or are joined together from single vanes or twin or triplet vane segments. While the latter construction involves high manufacturing costs plus considerable cooling-air leakage losses, these disadvantages are essentially eliminated when the vane assembly is manufactured as a fully integral component. Such vane assemblies are nevertheless encumbered by other disadvantages. For example, they come under high thermal fatigue loads induced by high radial and tangential temperature gradients. Difficulties are also encountered when incorporating small cooling air holes and when coating the components with hot gas corrosion-resistant surface layers and thermal barrier coatings.
A broad object of the present invention is to provide a vane assembly for a gas turbine which utilizes the advantages afforded by fully integral construction of low-cost manufacture and moderate leakage flows while avoiding its disadvantages, especially the high thermal fatigue.
This object is achieved according to the present invention by providing an arrangement wherein the other shroud is arranged separately and is capable of translation, and wherein an axial force is exerted on one of the two shrouds to urge the separately arranged shroud and axially chamfered tips of the vanes against each other by means of a clamping device lodging against one of an outer engine casing and inner turbine casing.
An important advantage provided by the present invention is that the vanes of the vaneaassembly are allowed to expand to suit the radial temperature profile and that the stresses that heretofore had occurred as a result of differing thermal expansions of vanes and inner or outer shroud are prevented. The vanes are allowed to radially expand in the absence of stresses, the expansion being kinematically balanced by the translation of the separately arranged shroud to continue the contact between the two components.
In a further aspect of preferred embodiments of the present invention the vanes an the fixedly connected shroud can be manufactured as an integral component to give simplicity in manufacture and achieve a nonleaking design.
In further certain preferred embodiments of the present invention the vanes can be fixedly connected to the outer or inner shroud as will suit the deiign of the flow passage or the combustion chamber. It will also be possible to arrange the clamping device such that the separate shroud is urged against the stationary vanes, or inversely that the separate shroud is fixedly connected to the casing while the clamping force is exerted on the shroud fitted with the vanes.
In certain preferred embodiments of the present invention the outer shroud is arranged separately. This provides an advantage especially in small gas turbine engines with reverse-flow combustion chambers, considering that the outer shroud here exhibits a more pronouncedly conical contour than the inner shroud. Should peculiarities of design give the inner shroud a more pronouncedly conical contour than the outer shroud, alternative use can be made of the embodiment featuring a separate inner shroud.
In certain preferred embodiments of the present invention the points of contact between the unrestrained ends of the vanes and the separate shroud are provided in the leading edge area of the vanes, for the reason that this is where larger contact and pressurefaces derive from the prevailing greater thickness of vane.
In certain preferred embodiments of the present invention a narrow radial clearance is provided in the vane trailing edge area between the vanes and the unrestrained shroud to permit temperature induced radial expansion of the vanes, so that in operation, contact at this point is avoided altogether or that contact forces, if arising, are kept small.
In certain preferred embodiments of the present invention aerodynamic losses are alleviated by ventilating the clearance with air taken from the chamber extending radially outside the outer shroud and admitted through holes. In an alternative or additional arrangement a portion of the stator cascade cooling air can be blown off.
In certain Preferred embodiments of the present invention the vane ends in the clearance area are suitably designed to reduce the flow around the vane ends.
In certain preferred embodiments of the prevent invention, wear protective coatings are deposited on the vane ends and/or the separate shroud to prevent fretting at the contact points.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.